For a semiconductor device in which an electrode is formed such as a solar cell, it is necessary to form an electrode with low resistance having a narrow occupied area on a light acceptance surface so as not to block light. Therefore, for the purpose of enlargement of electrode cross-section area with controlling the electrode width to a certain size, a solar cell or the like has been produced by forming a predetermined electrode groove on a substrate surface and then forming a buried electrode inside the groove. As examples thereof, buried-type electrode solar cells (buried-contact-type solar cells) have been developed and disclosed, for example, in Japanese Patent Application Laid-open (kokai) No. H08-37318 and No. H08-191152, and the like.
In the case of forming a finger electrode on a front surface of a solar cell, for example, a finger electrode formed by a screen printing method generally has approximately a width of 120 μm and a height of 15 μm. However, in the case of forming a buried-type electrode inside a groove as the above-described invention, a finger electrode having approximately a width of 30 μm and a depth of 60 μm can be formed by combining a plating method.
By forming a buried-type electrode (finger electrode) as described above, a solar cell with narrow electrode width and an electrode cross-section area being nearly equal to that of an electrode formed by a screen printing method can be produced. Thereby, without increasing resistance loss in the finger electrode portion, finger shadow loss is drastically reduced and the light acceptance area can be enlarged. Therefore, it becomes possible that the solar cell power is enhanced.
However, in the formation of a buried-type electrode by using a plating method as described above, in order to maintain the electrode formation condition to be constant, namely, in order to maintain a precipitation rate or a film composition to be constant, it is necessary that bath components such as a metal salt concentration of the electrode and a reducing agent concentration thereof are strictly analyzed and administered with pH and temperature, and it has been difficult to stably form an electrode having a desired quality. Moreover, in the case of using an electroless plating method, electrode forming cost is reduced. However, there has been a crucial disadvantage that formation of a predetermined cross-section area requires time and that productivity is very low. Furthermore, abnormal precipitation is easily caused against a minor defect on the device surface. Therefore, it is also difficult to pursue a beautiful physical appearance thereof.
With respect to such disadvantages, in Proceedings of 3rd World Conference on Photovoltaic Conversion, “Buried Contact Solar Cells on Multicrystalline Silicon with Optimized Bulk and Surface Passivation” or Japanese Patent Application Laid-open (kokai) No. 2001-223372, inventions for forming an electrode with the narrow electrode width by burying the electrode in a electrode groove by a screen printing method have been disclosed. However, for example, as the Japanese Patent Application Laid-open (kokai) No. 2001-223372 or the like, when a groove having a width of approximately 100 μm is formed on a semiconductor substrate and then an electrode material (electrode paste) for a solar cell which is generally used by a screen printing method is pressed into the groove, a binder component contained in the electrode material is burnt out in firing the electrode material, and the electrode material is constricted by sintering of silver grains. Thereby, there has been a problem that the electrode material is not sufficiently filled in the groove and simultaneously delamination of the electrode material is caused in the groove and the contact area to the semiconductor substrate is reduced. Furthermore, there has also been caused problems that a vacancy taken in the groove with the electrode material in the printing expands at a burst in the firing, or decomposition gas in the time of the burning-out loses vents and expands in the groove, and therefore, the electrode in the groove is partially broken. Such problems have increased series resistance of the solar cell and consequently have caused degradation of the power of the solar cell.
Moreover, in pressing the electrode material into the groove, it is devised to lower viscosity of the electrode material to be approximately 40-100 Pa·s and thereby the electrode material is made to easily flow in the groove. However, in this case, the electrode material leaks out of the groove and generates blur (seeping-out). If blur is generated in the electrode formation as described above, the electrode width of the finger electrode widens. Therefore, a problem that shadow loss is increased to degrade the solar cell power is caused. On the other hand, if a squeegee speed in the screen printing is simply enhanced in order to reduce the blur of the electrode, there has been a problem that patchiness is generated in the electrode in the groove and accordingly the finger electrode becomes easily broken and the solar cell power is degraded.